1. Field of the Invention
The invention relates to the preparation of cultured milk products and particularly to such products as quark and yogurt. The invention particularly relates also to membrane filtration of milk and similar liquid milk products containing coagulable protein, as a preliminary to reduce its mineral salt content and if desired concentrate its protein content before the protein is coagulated by incubation, for example, with lactic acid starters.
Membrane filtration is a technique developed recently to provide a cheap desalination process for rendering brackish water potable. The principle upon which it is based is the reversibility under sufficient pressure of the liquid flow through semi-permeable membranes due to reverse osmosis, solvent being removed from the salt solution under pressure through the membrane which accepts the smaller solvent molecules but resists the passage through it in the reverse direction of the larger salt ions.
The application of the membrane filtration principle has more recently been extended to effect the concentration of solutions both of inorganic and organic solutes, and particularly in the Food Industry.
The pressure applied to effect membrane filtration must exceed the osmotic pressure of the solution. Where this is considerable, as in the case of solutions of salts and solutes of similarly small dimensions, the process is usually known as reverse osmosis, whereas the term ultrafiltration connotes the application of the principle to solutions of substantially bigger and usually organic molecules, for example proteins.
Membranes suitable for effecting membrane filtration are usually cast from solutions of film-forming polymeric material, particularly cellulose esters, but including synthetic polymers, eg vinyl esters, polyolefins and acrylics. The membrane is necessarily flimsy and careful design of supporting equipment for it is necessary to avoid rupture in use.
The invention is concerned with the application of membrane filtration to the preparation of cultured milk products, notably quark and cheese. These products are prepared by incubation of lactic acid starters in milk and similar liquid milk products containing coagulable protein.